Three-dimensional hierarchical SnO₂ dodecahedral nanocrystals with enhanced humidity sensing properties

Highlights

• Three SnO₂ nanostructures with different morphologies were synthesized.

• Humidity-sensing properties were enhanced by tuning morphology and surface structure.

• 3D hierarchical SnO₂ DNCs exhibits fast response and recovery times.

• An impedance change of 4.2 orders was observed in 3D hierarchical SnO₂ DNCs sensor.

Abstract

One-step hydrothermal method was adopted to synthesize tin dioxide (SnO₂) nanostructures with different morphologies, i.e., three-dimensional (3D) hierarchical SnO₂ dodecahedral nanocrystals (DNCs), 3D hierarchical SnO₂ nanorods (NRs), and SnO₂ nanoparticles (NPs). The humidity sensors based on the as-synthesized nanostructures were produced to investigate the relationship between morphology and humidity sensing property. The nanosensor based on 3D hierarchical SnO₂ DNCs exhibited superior humidity sensing properties compared to those based on 3D hierarchical SnO₂ NRs and SnO₂ NPs. The enhanced sensing properties for SnO₂ DNCs are attributed to the peculiar 3D open nanostructures and high chemical activity of the exposed {101} facets. The 3D open nanostructures can promote the penetration and diffusion of water molecules, and the exposed {101} facets can improve the adsorption ability of water molecules. Furthermore, density functional theory (DFT) calculations were performed to further confirm that {101} facets can adsorb more water molecules than {110} facets. Our results demonstrate that morphology and surface engineering is an effective strategy to enhance the humidity sensing properties of nanomaterials.

Introduction

Since humidity has a great impact on the industrial production, storage of various goods and environmental monitoring, considerable investigations have been carried out to develop suitable humidity-sensitive materials for the fabrication of high-performance humidity sensors. Many types of functional materials such as metal oxides [1], polymers [2], [3] and inorganic/organic hybrids [4] have been utilized as humidity sensing materials. Among the various sensing materials, metal oxides have received intensive interest due to their unique properties such as chemical and physical stability, high mechanical strength and wide operating temperature range [5], [6], [7].

Tin dioxide (SnO₂) is a versatile semiconductor material which is commonly used in gas sensor [8], [9], lithium-ion battery [10] and solar cell [11]. Recently, SnO₂ has emerged as an important humidity sensing material due to its inherent chemical and physical stability [12] and easy adsorption of water in molecular or hydroxyl form [13]. However, SnO₂ humidity nanosensors displayed very long response and recovery times and low sensitivity [12], [14]. Hence, many scientific and technological efforts have been directed towards improving humidity-sensing performance of SnO₂ humidity nanosensors. For example, many semiconductor hetero-contact systems such as TiO₂-SnO₂ [15], WO₃-SnO₂ [16], and RGO-SnO₂ [17] were reported to possess enhanced sensitivity and fast response and recovery times. However, to our knowledge, little information is available in previous reports about enhancing the humidity-sensing properties of SnO₂ through tuning its morphology and surface architecture.

It has been found that the properties of SnO₂ strongly depend on its morphology and shape. For instance, Wang et al. [18] prepared hierarchical flower-like SnO₂ nanospheres assembled from short nanorods (NRs) for potential applications in gas sensing and lithium-ion battery. Zhao et al. [19] synthesized SnO₂ hierarchical architectures to investigate the relationship between the morphology and photocatalytic activities. However, few investigations focus on the correlation between morphology and humidity-sensing properties.

Herein, we synthesized SnO₂ nanostructures with different morphologies including three-dimensional (3D) hierarchical SnO₂ dodecahedral nanocrystals (DNCs), 3D hierarchical SnO₂ NRs and SnO₂ nanoparticles (NPs) through one-step hydrothermal process, and studied the performance of the humidity sensors based on different SnO₂ nanostructures. Interestingly, in our experiments the sensors realized with 3D SnO₂ nanostructures (DNCs and NRs) showed a response time of less than 4 s and a recovery time of less than 13 s, both significantly superior to the SnO₂ NPs. The fast response and recovery times are attributed to the pore channels present in the 3D SnO₂ nanostructures. Moreover, compared with 3D hierarchical SnO₂ NRs, 3D hierarchical SnO₂ DNCs displayed higher sensitivity with an impressive impedance change of 4.2 orders in 11–95% relative humidity (RH) range. The high sensitivity is ascribed to the exposed {101} facets of 3D hierarchical SnO₂ DNCs which can improve the adsorption ability of water molecules and enhance the humidity sensitivity. Furthermore, density functional theory (DFT) calculations are performed to prove that {101} facets can adsorb more water molecules than {110} facets. This work will pave the way to the development of promising practical humidity sensors.